Fix gen_localnet.py json exception

[sug0]

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.58%. Comparing base (5bf522f) to head (2604264).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4089   +/-   ##
=======================================
  Coverage   74.58%   74.58%           
=======================================
  Files         341      341           
  Lines      107484   107484           
=======================================
  Hits        80166    80166           
  Misses      27318    27318           

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